The invention relates to reinforcing fibres, in particular for fibre composite materials, their use, a process for their production, as well as a fibre composite material with reinforcing fibres based on carbon and to a process for its production.
Reinforcing fibres and fibre composite materials of the generic type are disclosed by German Patent Application 197 11 829.1, which is not a prior publication. The reinforcing fibres known from the latter are high-temperature resistant fibres in the form of short fibre strands. The fibre strands are impregnated with a binder suitable for pyrolysis. For this purpose, the fibre strands are immersed in the binder. The binder is subsequently made to set. Consequently, the fibre strands are held together and mechanically reinforced. The fibre strands are mixed with further binders and fillers, and the mixture is pressed to form a green compact, which is subsequently pyrolysed under a vacuum or inert gas to form a porous moulding. The fibre strands are thereby covered with a layer of carbon. The moulding is subsequently infiltrated with a silicon melt. This produces a C/SiC fibre composite material in which the fibre strands are embedded in an SiC-based matrix. The short fibre strands are embedded in a randomly distributed manner in the matrix, the individual filaments being largely retained. The coating of carbon has fully or partially reacted with the matrix material. As a result, the fibre strands are protected against the aggressive attack of the silicon melt. This fibre composite ceramic displays very good tribological properties and, in addition, is relatively inexpensive and simple to produce. It is suitable in particular for the production of brake discs and/or brake linings.
However, this material cannot withstand extremely high mechanical stresses, as occur for example as a result of large vehicle masses or extreme speeds. U.S. Pat. No. 4,397,901 discloses a further composite material with carbon fibres. The fibres are covered with a pyrolytic carbon layer which is then reacted directly with silicon. The finished composite article is then once again CVD-coated, using trichlorosilane with silicon carbide. A separate protective layer for the fibres is not disclosed.
The object of the invention is therefore to provide a reinforcing fibre and a composite material of the above-mentioned type which offer even higher strength and better quasi-ductility of the component, but nevertheless are easy and inexpensive to produce and are therefore suitable for series production.
Pyrolytic carbon is understood here to mean both a pyrolytic dip coating and a carbon layer deposited from the gas phase.
The reinforcing fibres or fibre strands according to the invention are thus in each case coated with two additional layers. The lower layer, applied directly on the fibre or the fibre strand, is of pyrolytic carbon. Applied to this layer is a dip coating known per se of a pyrolysable binder. These fibres or fibre strands are worked in the way described above into a green compact, which is then pyrolysed to form a porous moulding. During the impregnation of the porous moulding with liquid silicon, the carbon layer originating from the resin coating acts as a xe2x80x9csacrificial layerxe2x80x9d. The liquid silicon reacts with this outermost layer to form a silicon carbide. This represents a diffusion barrier for the liquid silicon, which consequently cannot penetrate any further into the fibre or fibre strand. The lower-lying layer of pyrolytic carbon and the reinforcing fibres or fibre strands are not attacked. Rather, the layer of pyrolytic carbon acts as a graphitic structure with sliding properties, i.e. the fibre or the fibre strand can slide along on this structure.
The composite materials containing fibres or fibre strands treated in this way are therefore distinguished by very good mechanical properties and particularly high strength. The additional layer of pyrolytic carbon brings about an optimum bonding of the reinforcing fibres to the matrix. They have a crack-diverting effect and can slide in a longitudinally movable manner, which brings about the good results of the strength and three-point bending tests. Fibre pullout effects are possible. This results in very good mechanical properties of the fibre-reinforced composite material.
The use of these reinforcing fibres according to the invention in fibre composite materials, even in small amounts as a proportion of the total fibre volume, allows the values for strength and elongation to be significantly increased, as can be demonstrated for example in the three-point bending test. The remaining parameters are not adversely affected by this.
Thus, if the mechanical loads for the component are extremely high, strength and elongation values can be further increased. In the case of particularly high mechanical stresses, it is possible by the present invention to adapt known processes for the inexpensive production of fibre-reinforced composite ceramic to the extent that the material offers particularly high external strength with significantly increased internal quasi-ductility of the component.
The process for producing the reinforcing fibres according to the invention is distinguished in that carbon fibres are initially coated with pyrolytic carbon. The fibres are subsequently provided with pyrolysable polymer material.
The coating may take place on the one hand by dip coating, for example immersion in a pitch bath. This process is suitable in particular for long fibres or continuous fibres. On the other hand, the coating may take place by depositing on the fibres a carbon layer from the gas phase. An example is CVD coating with hydrocarbons, for example with methane, in a reactor. This process is suitable both for short fibres and for long fibres or continuous fibres.
The use of pitch has the further advantage that crystalline carbon, which reacts much more slowly with liquid silicon than a layer of amorphous carbon, as is produced when using a phenolic resin for example, is produced as the pyrocarbon layer. This further reinforces the diffusion barrier for the amorphous carbon.
A further refinement of the invention provides that the long fibres or endless fibres are cut up after coating and before processing into a green compact.
Fibre bundles treated in the way according to the invention preferably comprise approximately 1000 to 14,000 individual fibres with average diameters of approximately 5 to 10 xcexcm and a length of approximately 10 to 30 mm. In this way, commercially available fibre strands can also be used. This makes low-cost production possible.
All common reinforcing fibres can be used. Carbon fibres are preferred. However, in principle other high-temperature resistant fibres, such as silicon carbide fibres or Si/C/B/N-based fibres are also suitable, as are metal fibres and glass fibres. Titanium fibres and also aramid fibres are well suited.
Very good results are obtained if only fibres treated in this way are used for producing the green compacts. Positive effects can be measured, however, from a fibre content of as low as approximately 10%, in particular approximately 15% as a proportion of the total fibre volume. A content of approximately 40% as a proportion of the total fibre volume of the green compact is particularly preferred. With this proportional content, the cost-benefit ratio is particularly favourable.